Ratio of l-(+)- and d-(−)-Lactic Acids Produced by Enterococcus faecalis Changes Depending on the Culture pH

Abstract

Enterococcus faecalis (E. faecalis) has been associated with the specific production of l-(+)-lactic acid. However, in this study, d-(−)-lactic acid production by E. faecalis was observed under specific pH conditions. E. faecalis PR31 exhibited a significant amount of d-(−)-lactic acid under a stirring culture in MRS broth at pH 4.5, 5.8, and 6.0, and the contents of d-(−)-lactic acid were 45.1, 35.9, and 36.2%, respectively. When the cell suspension prepared at a pH of 6.0 was reacted with l-(+)- or d-(−)-lactic acid, d-(−)- or l-(+)-lactic acid was produced, respectively, in a time- and dose-dependent manner. Therefore, this phenomenon of d-(−)-lactic acid production in PR31 was suggested to be due to the activation of the larA gene encoding lactate racemase that is present in PR31. However, even in the E. faecalis-type strain NBRC 100480, which contains neither larA nor vanH, encoding d-(−)-lactate dehydrogenase VanH, d-(−)-lactic acid was also produced at specific pH values. Therefore, the production of d-(−)-lactic acid in NBRC 100480 was thought to occur not via the activation of larA. The biological significance of d-(−)-lactic acid production in E. faecalis depending on the pH and the detailed underlying mechanism, including whether it is the same in PR31 and NBRC 100480, remain to be elucidated in future studies.